Rotary screw machine having thrust balancing means

ABSTRACT

The invention relates to a rotary screw machine in which the shaft journals (20, 22) of the rotors (10) are journalled in a main thrust bearing (24) and a thrust balancing bearing (26). The thrust balancing bearing (26) is preloaded by springs (36) and by fluid pressure means (40, 44). According to the invention the fluid pressure means (40, 44) can excert a force on the thrust balancing bearing (26) in either axial direction. This increases the possibility for an optimal distribution of the forces on the thrust bearings (24, 26) at various running conditions.

BACKGROUND OF THE INVENTION

The present invention relates to a rotary screw machine having at leastone pair of rotors which are affected by forces from the working fluidin a first axial direction, at least one of the rotors being providedwith shaft journals supported by bearing means, including main thrustbearing means and thrust balancing bearing means having a rotating ringand a stationary ring and being provided with thrust balancing means,said thrust balancing means including spring means acting on saidstationary ring in said first direction and fluid pressure means actingaxially on said stationary ring.

A compressor of this kind is known from U.S. Pat. No. 3,388,854. In thatdisclosure, the stationary ring of the thrust balancing bearing ispre-loaded by a spring 35 acting in the same direction as the axial gasforces on the rotors. The stationary ring also abuts a fluid actuatedpiston 36, through which a force can be applied to the ringcounteracting the force from the spring. The pressure chamber 37,through which a pressure can be applied to the piston 36 can beconnected to a pressure source, i.e. the outlet channel. This is thecase under normal operating conditions, whereby the thrust load will bedistributed to the thrust bearings on both ends of the rotor. When thecompressor is idling, the pressure chamber is relieved to the atmosphereso that only the spring pre-loads the stationary ring. Through thisknown device, in many cases, a satisfactory distribution of the axialforces on the thrust bearings is attained, but it entails still somelimitations regarding an optimal distribution of these forces.

The object of the present invention is to improve the known thrustbalancing device in order to reach such a force distribution on thethrust bearings so that the resultant force on each thrust bearing fallswithin a more narrow range, thereby increasing the possibility to meetthe requirements for a sufficient working life for each of the thrustbearings.

SUMMARY OF THE INVENTION

According to the present invention, this object has been attained inthat a device of this kind is provided with means for regulating theaxial direction of the force exerted by the fluid pressure means.

Each thrust bearing has to be loaded within a certain range, where themaximum force is determined by the working life of the bearing, and theminimum force has to be large enough to avoid sliding of the bearingballs in the rings. With a balancing device according to the invention,the possibilities to attain a force distribution for the bearings sothat the force on each bearing falls within this range, will increasedue to the fact that the force on the stationary ring of the thrustbalancing bearing can be either the sum of the fluid pressure force andthe spring force, the spring force along or the difference between thefluid pressure force and the spring force. By having these differentalternatives for loading the stationary ring of the thrust balancingbearing, it will be possible to adapt this loading to the differentrunning conditions of the machine; starting, idling, working at lowpressure and working at full pressure. During these various runningconditions, the external axial force on the rotors, comprised mainly byforces from the pressure of the working fluid but also by forces fromdriving and timing gears, are of different strength.

With the earlier known technique, where the fluid pressure force on theouter ring either is zero or acts contrary to the spring force, thepossibility to adapt the loading of the ring to the various runningconditions are more limited, and with that the possibility to keep theforces within the prescribed ranges.

The fluid pressure means preferably takes the form of a pressurechamber, the pressure of which acts on a surface on the stationary ring.The regulating means selectively connects the pressure chamber witheither overpressure, atmospheric pressure or underpressure. The machineis particularly intended to be used as a compressor, in which case theoverpressure source preferably is the outlet channel thereof and theatmospheric pressure source as well as the underpressure source is theinlet channel.

In a preferred embodiment the means for selectively connecting thepressure chamber with a fluid pressure source includes a two-way valveregulated by the outlet pressure of the compressor and connecting thechamber either with the outlet channel or the inlet channel of thecompressor. These means preferably also include variable throttlingmeans in the inlet channel of the compressor.

It might be convenient to fix the stationary ring in an axially movablemember through which the spring force and the fluid pressure force aretransmitted to the ring.

The invention can advantageously be applied to a multistage compressor,in which case the high pressure source can be the flow path of theworking fluid at a point anywhere between the outlet port of the firststage and the outlet port of the last stage, preferably in the inletchannel of any of the stages later than the first stage.

The invention will be explained through the following detaileddescription of a preferred embodiment thereof and with reference to theaccompanying drawing showing a schematic section through the male rotorof a compressor according to the invention. Details of the compressornot being essential for the understanding of the invention are omittedfrom the drawing for the sake of clarity.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic section through the male rotor of a compressoraccording to the present invention;

FIG. 2 shows a pair of screw rotors of the compressor according to thepresent invention; and

FIG. 3 is a block diagram illustrating the present invention as appliedto a two-stage compressor.

DETAILED DESCRIPTION

In FIG. 1, 10 represents the male rotor of a rotary screw machine. Themale rotor cooperates with a female rotor (as shown in FIG. 2) throughhelical lobes and grooves on the rotors in a manner well-known. Throughchevron-shaped working chambers formed by the rotors and the surroundingcasing, a gaseous fluid, e.g. air, is compressed. The air is supplied tothe compressor from an inlet channel 16 through an inlet port 12, andthe compressed air leaves the compressor through an outlet port 14 to anoutlet channel 18.

The rotor 10 is provided with shaft extensions or shaft journals 20, 22at its ends, through which the rotor is journalled in thrust bearings24, 26. Elements like journal bearings, shaft sealings, drivingconnection and timing gears normally also are present, but in order toelucidate the invention they are left out from FIG. 1.

The arrow F represents the external axial force acting on the rotor 10during operation. This force normally is directed to the right in FIG.1, i.e. towards the low pressure end of the compressor, which is definedas the positive direction. The force F comprises the force acting on therotor due to the pressure difference between the high pressure end andthe low pressure end of the compressor and the forces coming from thedriving and timing gears. The force due to the pressure differencenormally is dominating and is always in the positive direction. Theresultant of the forces from the driving and timing gears acts in thenegative direction, but since this force is much smaller, the totalforce F normally is positive.

The external axial force F is taken up by a main thrust bearing 24 atthe high pressure end and a thrust balancing bearing 26 at the lowpressure end. The main thrust bearing 24 abuts a part 32 of the casingand is capable of taking up forces in the positive direction.

The thrust balancing bearing 26 has its stationary ring 30 fixed in anaxially movable member 38. Although shown as a single unit, the member38 is comprised of two parts to make the assembly possible. Springs 36supported by a part 34 of the casing act on member 38 with a force inthe positive direction. Also acting on the member 38 is fluid pressurewithin a sealed chamber 44. The fluid pressure in this chamber 44 actson a pressure surface 40 of the member 38, and if the pressure in thechamber 44 is above atmospheric pressure, a force in the negativedirection occurs which thus counteracts the force from the springs 36.If the pressure in the chamber 44 is below atmospheric pressure asuction effect on the member 38 is attained since the pressure on theother side thereof always is about atmospheric pressure. In this casethe fluid pressure force on the member 38 will be in the positivedirection, i.e. in the same direction as the force coming from thesprings 36. If the pressure in the chamber 44 is of atmosphericpressure, only the spring force will pre-load the stationary ring 30.Through a connection pipe 46 and a two-way valve 48 the chamber 44 canbe connected either with the outlet channel 18 through a pipe 50 or withthe inlet channel 16 through a pipe 52. The position of the two-wayvalve 48 is regulated by means sensing the outlet pressure. By means ofa throttle valve 54 in the inlet channel 16, the incoming air can bethrottled, whereby underpressure will develop in the inlet channel 16downstream of the throttle valve 54.

For a certain thrust ball bearing there exists a maximum force F_(max)that can be allowed with respect to its running life. There is also aminimum force F_(min) required in order to avoid sliding of the balls inthe races. The range F_(min) to F_(max) thus determines the allowableforce on the thrust bearing.

How the described device makes it possible to distribute the axialforces to the main thrust bearing 24 and the thrust balancing bearing 26so that the force on each of them will remain within the allowable rangeat different running conditions will be explained by the followingexample.

The bearing used for the main thrust bearing 24 has a F_(min) =1100 Nand a F_(max) =1800 N, and the corresponding valves for the thrustbalancing bearing are 300 N and 800 N, respectively. The main thrustbearing 24 is capable of taking up forces in the positive direction,whereas the thrust balancing bearing 26 is of a kind allowing load ineither direction. The total spring force, F_(S) is 400 N.

At idling, the throttle valve 54 is in its closed position (shown bybroken lines in the figure) thereby creating an underpressure inletcondition. The pressure at the outlet will be about atmospheric. At thisoperating conditon the external force on the rotor was 422 N in thepositive direction. The two-way valve 48 is in a position where thesealed chamber 44 is connected to the inlet channel 16 downstream of thethrottle 54. Since the underpressure in the inlet channel thereby istransmitted to the sealed chamber 44, there will be a suction force onthe movable member 38, which means that the direction of the force ispositive. This force, F_(B) will be 316 N. The total axial load on thethrust balancing bearing 26, F_(TB) coming from the spring force and theforce from the underpressure thus will be 400+316=716 N. The load on themain thrust bearing 24, F_(T) will be the sum of the external force andthe resultant force on the thrust balancing bearing 26, with which arepositive. Thus, F_(T) =422+716=1138 N.

When the compressor is loaded, the throttle 54 is set in its openposition. When working at a certain low delivery pressure the externalforce, F was found to be 1280 N. Also under this working condition thevalve 48 connects the sealed chamber 44 to the inlet channel 16. Sincethe pressure in the inlet channel 16 now is about atmospheric pressure,there will be neither over- nor underpressure acting on the pressuresurface 40 of the movable member 38. Consequently the only force exertedon the thrust balancing bearing 26 will be that from the springs 36,F_(S) =400 N. The load on the main thrust bearing 24 thus will be1280+400=1680 N.

When working at full delivery pressure, the external force F, was foundto be 2248 N. In this case the two-way valve 48 is in a positionconnecting the sealed chamber 44 to the outlet channel 18, so thatoverpressure will prevail in the sealed chamber. This creates a force of892 N in the negative direction on the member 38, which is counteractingthe force from the springs 36. Consequently there will be a load on thethrust balancing bearing 26 in the negative direction amounting to F_(B)-F_(S) =892-400=492 N. The load on the main thrust bearing 24 thereforewill be 2242-492=1750 N.

The different forces occurred in the above described example are puttogether in the table below:

    ______________________________________                                                          low del.  full del.                                         unloaded          pressure  pressure                                          ______________________________________                                        F       422           1280       2242                                         F.sub.B 3.16           0        -892                                          F.sub.S 400           400        400                                          F.sub.TB                                                                              716           400       -492                                          F.sub.T -1138         -1680     -1750                                         ______________________________________                                    

As can be seen from the table, the forces on the thrust bearings F_(TB)and F_(T) all the time will be within the allowed range 300-800 N and1100-1800 N, respectively. This is a direct consequence of theinvention, making it possible to attain a force from the fluid pressuremeans which cannot only be zero or directed in a first direction, butalso in a second direction. Without introducing the latter feature, thiscould not be achieved.

As shown in FIG. 3, the invention can be applied to a multi-stagecompressor, each stage containing one pair of rotors, wherein the fluidpressure source having a pressure above atmospheric pressure is the flowpath of the working fluid in a point anywhere between the outlet port 14of the first stage (stage I) and the outlet port of the last stage(stage II). In a preferred arrangement, the point is located in theinlet channel 16 of any stage later than the first stage. The chamber 46is selectively connected with a fluid pressure source by means of thetwo-way valve 48 regulated by the outlet pressure of the first stage(stage I) of the multi-stage compressor, and the variable throttlingmeans 54 in the inlet channel (16) of the first stage of the multi-stagecompressor.

I claim:
 1. A rotary screw machine comprising:a casing; at least onepair of rotors arranged in said casing and being subject to forces froma working fluid in a first axial direction of said rotors; shaftjournals provided on at least one of said rotors; bearing means forsupporting said shaft journals relative to said casing, said bearingmeans including main thrust bearing means and a thrust balancing bearingmeans; said thrust balancing bearing means including a rotating ring anda stationary ring and being provided with thrust balancing means, saidthrust balancing means including spring means acting on said stationaryring in said first axial direction and fluid pressure means for actingaxially on said stationary ring, said fluid pressure means including asealed chamber and a surface, facing said sealed chamber, said surfacebeing defined on a member rigidly connected to said stationary ring; andregulating means including means for selectively connecting said sealedchamber with a fluid pressure source having a pressure above atmosphericpressure, at atmospheric pressure, or below atmospheric pressure.
 2. Therotary screw machine according to claim 1, operating as a compressor,further comprising:an inlet channel provided with a variable throttlingdevice therein; and an outlet channel; wherein said fluid pressuresource having a pressure above atmospheric pressure comprises saidoutlet channel; wherein said fluid pressure source having atmosphericpressure comprises a portion of said inlet channel downstream of saidvariable throttling device; and wherein said fluid pressure sourcehaving a pressure below atmospheric pressure comprises a portion of saidinlet channel downstream of said variable throttling device.
 3. Therotary screw machine according to claim 2, operating as a compressor,wherein said regulating means for selectively connecting said sealedchamber with a fluid pressure source includes a two-way valve regulatedby an outlet pressure of the compressor and connecting said sealedchamber with one of said outlet channel and said inlet channel of thecompressor.
 4. The rotary screw machine according to claim 1, comprisinga plurality of pairs of rotors, each pair of rotors operating as a stagein a multi-stage compressor, wherein said fluid pressure source having apressure above atmospheric pressure comprises a flow path of the workingfluid at a point between an outlet port of a first stage and the outletport of a last stage of said multi-stage compressor.
 5. The rotary screwmachine according to claim 4, wherein said fluid pressure source havinga pressure above atmospheric pressure comprises a flow path of theworking fluid at a point located in an inlet channel of any stage laterthan the first stage of said multi-stage compressor.
 6. The rotary screwmachine according to claim 5, wherein said regulating means forselectively connecting said sealed chamber with a fluid pressure sourceincludes:a two-way valve regulated by an outlet pressure of the firststage of said multi-stage compressor; and a variable throttling devicein the inlet channel of said multi-stage compressor.
 7. The rotary screwmachine according to claim 4, wherein said regulating means forselectively connecting said sealed chamber with a fluid pressure sourceincludes:a two-way valve regulated by an outlet pressure of a firststage of said multi-stage compressor; and a variable throttling devicein an inlet channel of said multi-stage compressor.